Oscillator



2 Sheets-Sheet 2 Jude/e301" PAUL WEATHERS afim V4550! nay Feb. 17, 1948.P. WEATHERS OSCILLATOR Filed. Dec. 22, 1945 Patented Feb. 17, 19482,436,129 QSCILLATOR Paul Weathers, Haddon Heights, N. J., assignor toHerbert K. Neuber, Philadelphia, Pa.

Application December 22, 1945, Serial No. 636,702

17 Claims. (Cl. 179-1715) The present invention relates to oscillatorsof the electronic tube type, and more particularly to self-excited,electronic tube, modulated oscillators, and has for its primary objectto provide an improved oscillator of the character referred to, which isresponsive to extremely small changes in an electrical characteristic ofa control circuit therefor, such as a relatively small capacity,inductance, or resistance variation, occurring at a remote or extendedend of said control circuit, to produce a greatly enhanced control ormodulation effect on said oscillator and a corresponding relativelylarge and amplified change in oscillator anode current or outputpotential.

It is also an object of this invention, to provide an improved methodand means for controlling an electronic tube oscillator whereby theanode current or output potential thereof may be varied with a highdegree of sensitivity and over a relatively wide range in response toextremely small changes in capacity, inductance or resistance in acontrol circuit coupled with the oscillator, and whereby the variablecapacity, inductance, or resistance referred to may be located at theend of a relatively long extension of the control circuit, such as atransmission line and may, therefore, represent only a relatively smallportion of the total overall capacitance, inductance, or resistance ofthe circuit, without impairing the sensitivity and control range.

It is a still further object of the present invention, to provide animproved oscillator of the selfexcited, electronic-tube type which takesadvantage of the Miller, or grid-circuit frequency-variation effect,with variation in control grid bias, as described in chapter 7 of"Radiotron Designers Handbook, third edition, a publication of RCAManufacturing Co., Inc., Harrison, N. J in providing further depth andamplitude to the modulation action thereon in response to extremelysmall or minute variations in an electrical characteristic of a controlcircuit therefor.

It is a further object of this invention, to provide dual action orpush-pull control in the modulation of an electronic-tube oscillator,through a control connection involving a relatively long transmissionline and by means of substantially minute variations in capacity,inductance, or resistance at the end thereof.

Because of the permissible extension of the control circuit through atransmission line and the relatively small circuit elements required foreffecting a relatively wide control action on the modulation or anodecurrent change, a modulated oscillator circuit embodying the inventionis particularly well adapted for use in high fidelity phonograph recordreproduction systems.

In accordance with the invention, the modulation source or pick-up maycomprise a small, light-weight push-pull variable capacitor mounted onthe end of a light-weight tone arm, whereby record wear is reduced to anabsolute minimum, and the oscillator and associated circuits may becoupled thereto through a shielded or unshielded transmission line ofseveral feet in length without introducing undesired modulation effects,and without, in any way, limiting the frequency range of response of thepick-up or the amplitude of the controlling efiect upon the oscillatorand the resulting signal output therefrom.

Furthermore, because of its sensitivity to relatively small electricalcontrolling effects, an electronic tube oscillator circuit embodying theinvention is particularly adapted for use in detecting small variationsin the content of flowing fluids and for registering the approach anddeparture of a moving body with respect to a protected area. Thus, anoscillator circuit embodying the invention may readily be adapted fordetecting foreign matter in the flow of gasoline, oil or water, forexample, and for the protection of safes, windows, and doors in an alarmsystem or the like, as well as for phonograph record reproduction,microphone sound pick-up and the like.

In accordance with the invention, two high frequency circuits, tuned tosubstantially the same frequency, are coupled to permit the transfer ofenergy from the one to the other, the one or first tuned circuit beinglocated in the anode circuit of an electronic tube oscillator and theother, or second tuned circuit, being coupled preferably by looseinductance coupling, with the grid circuit of said oscillator andarranged to provide differential feed-back therewith. The grid circuitis tuned to a slightly lower frequency than said first and secondcircuits and of the same order.

Further, in accordance with the invention, a balanced-to-cathode orground transmission line or control circuit is connected with the secondcircuit, in such a manner that it is extremely sensitive to capacity,inductance, or resistance changes at its terminal end and operates totrigger the flow of energy from the first to the second circuit, andthence to the grid circuit of the oscillator by the couplinghereinbefore referred to, thereby to provide by a dynamic feed-backaction on the grid circuit, which is amplified by the Miller effect, agreatly enhanced variation in anode current or output potential, andrelatively deep modulation efiect upon the oscillator in response tovariations of said capacity, inductance, or resistance.

The invention will, however, be more fully understood from the followingdescription when considered in connection with the accompanyingdrawings, and itsscope will be pointed out in the appended claims.

In the drawings:

Figure 1 is a circuit diagram of electronic tube modulated oscillatorembodying the invention in a present preferred form;

Figure 2 is a circuit diagram illustrating the electrical constructionof a circuit element of Figure 1;

Figures 3 and 4 are graphs showing curves illustrating certain operatingcharacteristics of the circuit of Figure 1;

Figures 5, 6 and '7 are circuit diagrams showing detail modifications ofa portion of the circult of Figure 1 in accordance with the invention;

Figure 8 is a schematic circuit diagram of electronic tube modulatedoscillator and associated amplifier circuits embodying the invention ina practical form for phonograph record reproduction; and

Figures 9 and 10 are cross-sectional views, and substantially full size,of certain of the circuit elements of Figure 8.

Referring to Figure 1, I2 is an electronic oscillator tube of thethermionic type having an anode or plate I3, a control grid 14, and aheated cathode l5. Between the grid and the cathode is connected a tunedgrid circuit 16 comprising a variable tuning inductance I1 and shunttuning capacity indicated at l8, comprising mainly the reflectedgrid-to-plate capacity, Cgp, of the tube, together with stray capacityand the grid-tocathode capacity of the tube.

A grid resistor l9 and shunt grid capacitor 23 are connected between thegrid and the inductance H at the high potential side of the circuit IS.The cathode is connected to ground through a lead 2|, and a grid circuitreturn lead 22 to cathode from the tuning inductance I1, is provided atthe low potential side of the tuned circuit l6.

Since the capacity represented at [8 is the sole tuning capacity for thecircuit [6, the in- I ductance element at H may be made relatively highin inductance value and the tuning adjustment of the circuit l6 may thenbe accomplished by varying the inductance l1, preferably by movable ironcore tuning means as indicated at 23.

A similar tuned circuit 25 is connected between the anode and thecathode. This comprises a variable tuning inductance 26 connected to theanode through an anode circuit lead 21 and connected to the cathode andground through a lead 28 and a by-pass capacitor 29. The shunt tuningcapacity for the circuit 25 is provided by the combined plate-to-cathodeand the plate-togrid capacities of the tube, indicated at 30.

The tuned anode and grid circuits are of the high inductance type,adapted for tuning to a relatively sharp resonance peak. As in the caseof the grid circuit, the anode circuit is tuned to desired resonance byvarying the inductance 26, preferably, as indicated, by means of amovable iron tuning core indicated at 3|.

Anode current is supplied to the oscillator from a positive supply leadindicated at 32, through an output coupling impedance provided by aresistor 33, and a radio frequency choke coil 34 connected with the lead28. A low or audio frequency signal output terminal 35 is providedbetween the'choke coil 34 and the output resistor 33 in connection withthe high potential end of the latter. This is for the purpose ofderiving signal output from the impedance 33 as the resultof anodecurrent variations at the modulation frequency. Other output connectionmay be made with the anode circuit to derive therefrom any desiredelectrical output effect. For example, the tuned anode circuit 25 may becoupled to an output circuit indicated at 35 through a couplingcapacitor 3'! to provide modulated radio frequency signals for furtherampl'fication, detection and utilization as in any conventional radioreceiver circuit.

When the grid and anode circuits l6 and 25 are tuned to resonance byadjustment of the respective cores 23 and 3|, the tube 12 will oscillatebecause of energy feed-back through the grid-to-plate capacity, Cgp,from the circuit 25 to the circuit 16. When the tube oscillates, theanode current flow through the resistor 33 decreases because of anincrease in negative bias on the oscillator grid I4 established by thegrid current flow through the grid resistor [9, until a steady state ofoscillation is established. The oscillator is thus self-excited andtends to oscillate at a fixed frequency until the feed-back or energyapplied to the grid circuit is varied.

Difierential variation of feed-back for modulation or other control ofthe anode current and the oscillator signal output, is provided througha third tuned circuit 40, comprising an inductance winding 4| having acenter tap 42 connected or coupled with the anode I3 through a couplingcapacitor 43 and a feed-back circuit 44. The terminal ends of theinductance winding are connected to a transmission line forming part ofa control circuit and comprising a pair of closely associated insulatedleads 45 and 46, preferably twisted together and inclosed within aconducting shield 41, which is connected as indicated at 48 to thecathode return lead 22 of the oscillator grid circuit [6 and ground forthe system.

With this arrangement, it will be seen that the substantially fixedcapacity existing between the twisted leads 45 and 4B is effectivelyconnected in shunt across the tuning inductance 4| as indicated at 49,thereby providing a shunt tuning capacity for the circuit 40. When,furthermore, a shield is provided for the transmission line, additionaltuning capacity effectively across the inductance 4| is provided by thestray capacity between each of the twisted leads of the cable and theshield 41, the components of which are indicated at 50 and 5|respectively. The total tuning capacity across the inductance 4|,therefore, is that indicated at 49, plus the capacities indicated at 50and 5| in series.

The capacities 50 and 5| are also connected through th feed-back controlcircuit efiectively in parallel relation to the oscillator tuninginductance 26 since the two halves of the winding M are difierentiallyconnected in the feedback path and inductively neutralize each other,and therefore, as the length of the cable is increased the capacities toand 5| likewise increase in value and tend to tune the anode circuit toa lower frequency. At the same time, the effective capacity 49 betweenthe leads 45 and 4B of the cable is increased, thereby correspondinglylowering the tuning response of the feed-back con-- trol circuit 49, andcausing the two circuits to remain substantially in resonance one withthe other regardless of the extension of the control cable within normallimits. In any case, how-= ever, the tuning change for any length of thecable as required, which may tend to provide an appreciable frequencydifference, may be corrected by adjustment of the tuning of theinductance 26 in the anode circuit by means of the tuning core 3|.

Since the cable comprises a closely twisted or associated pair, thevalue of the capacity 49 is substantially fixed, while shifting of thecable in the shield or movement with respect to the system ground andcathode connection as by movement or vibration of the cable and shield,changes both of the capacity values indicated at so and 5| substantiallyequally. Therefore, as a result, the overall change in capacity balanceis effectively zero and no undesired modulation or noise effect isproduced upon the system, as will hereinafter be described.

The terminal end of the transmission line is connected to a suitablevariable control or modulation source providing inversely variable, andpreferably normally balanced, impedance paths to ground or cathode forthe control circuit leads 45 and 46. In the present example, themodulation source is provided by a push-pull variable capacitor havingfixed plates 55 and 56 connected respectively with the terminal ends ofthe leads 46 and 45, and a plate or electrode 51 located between thefixed plates and pivoted as at 59 for example, for oscillation betweensaid plates to increase the air or dielectric gap on one side andcorrespondingly decrease the air or dielectric gap on the other inresponse to movement imparted thereto by a phonograph record pick-upstylus 60, representing any suitable means for imparting push-pullcontrolling action to the balanced capacitor arrangement. The variableimpedance path to ground or oscillator cathode from each electrode 55and 56 is completed by connecting the movable plate 51 as at the pivot59, to ground through a lead 6| connected with the shield 41.

Referring to Figure 2, along with Figure 1, a present preferredconstruction for the tapped coil or inductance 4| is shown, in which thetwo sections or portions on either side of the midtap 42 are inductivelyequal and comprise parallel conductors preferably wound in a bi-filar ordouble lead arrangement simultaneously and connected in series aiding asindicated. The total overall inductance of the winding 4| is madeessentially equal to that of the inductance 26 in the anode circuit. Theoperational effect is to provide a balanced feed-back coil or inductancewhich may be tuned overall to the same frequency as the anode circuit.The tap 42 is thus at the electrical or inductive center of the winding4|, and therefore, a current introduced at the tap 42 will flow inopposite directions through the winding 4| to the leads 45 and 46, andprovide opposing feed-back or inductive coupling efiects in the circuit40 and upon any associated inductive winding such as the inductance inthe grid circuit.

The two portions of the inductance 4| preferably are twisted togetherbefore winding to further insure a balanced feed-back effect with thesame current flow in each portion and essentially the same straycapacity coupling between each portion and the inductance H of the gridcircuit. Loose coupling is preferably provided in order that the straycoupling mry be reduced to a minimum and to permit the grid circuit tovary in frequency independently of the frequency of the circuit 40 andof the circuit 25, as will hereinafter be described.

With the balanced feed-back arrangement described, it will be seen thatas the impedance to cathode or ground return circuit of the system atthe terminal ends of the transmission line is varied by movement of thebalanced capacitor armature 51 between the fixed electrodes 55 and 56,feed-back from the anode circuit 25 to the grid circuit IE will beincreased and decreased correspondingly by reason of the differentiallyderived currents reducing oscillation amplitude when unbalanced in onedirection and increasing oscillation amplitude when unbalanced in theopposite direction through the winding 4|, thereby increasing anddecreasing the effective feed-back to the grid circuit and the amplitudeor intensity of the oscillation voltage therein. The correspondingvariation in the bias voltage across the grid resistor l9, and in theanode current flow through the output resistor 33 in proportion thereto,is greatly amplified by reason of the push-pull feed-back action and theMiller effect as will be seen from a further consideration of thecircuit of Figure 1 and the graphs shown in Figures 3 and 4.

Referring to Figure 3, the relation between grid bias voltage variationwith capacity change at 55-56-51 and resulting anode or plate currentchange is illustrated by the curve 65. Normally, the adjusted value ofgrid resistor IS, the feed-back through the grid-to-plate capacity andthe resulting grid bias is such that the anode current is adjusted to avalue represented at the point 66 with steady-state oscillation at afixed frequency.

The differential feed-back through the inductance 4| under control ofthe inversely variable or push-pull impedance at 5556-51 is such thatthe grid bias Varies to effect an anode current variation between limitsindicated at 61 and 68, the latter being just below the upper knee ofthe plate-current grid-bias curve in order that the anode currentvariations may be along a linear portion of the curve and follow withoutdistortion the modulation variations at the modulation source.

The tube thus operates in a high gain control range, with the anodecurrent at a normal minimum consistent with permitting a desiredvariation in anode current without distortion. Thus, the oscillator mayprovide weak oscillations without losing effectiveness and withoutradiating energy which would cause objectionable interference with otherelectronic apparatus in the immediate vicinity.

Referring now to the arrangement for utilizing the Miller effect in thegrid circuit and resulting bias variation to provide a wider variationin anode current and modulation output than would otherwise be possiblewith the pushpull feed-back control, it will be seen that since thegrid-to-plate capacitance, Cgp, is variably amplified by variations inthe bias voltage of the oscillator, and in effect shunts the gridinductance at l8, as previously described, it is obvious that theresonance of the grid circuit I6 will drop in frequency with a decreasein the strength of oscillations, and conversely will rise in frequencywith an increase in the strength of oscillations as provided by themodulation feed-back through the coil 4| in response to variations inthe terminal impedance of the transmission line.

It has been found that to obtain the aiding action of the Miller effect,the grid circuit must be tuned to a mean frequency of the order of andslightly below the resonance frequency of the anode circuit, that is,the anode circuit 25 is tuned to a higher frequency than the gridcircuit I6, so that as the strength of oscillation increases in the gridcircuit I6, the resonance frequency of the latter circuit rises and moreclosely approaches that of circuit 25, thus increasing the strength ofoscillations further, by reason of its closer approach to resonance withthe circuit 25, until a condition of equilibrium is reached. Conversely,this action aids in decreasing the strength of oscillations when thefeed-back through coil 4| is negative and opposes feed-back grid-toplatecapacitance Cgp. This action will further be described hereinafter whenconsidering the operation of the system.

With the foregoing arrangement and frequency relation between the tunedanode and grid circuits, it will be seen that as the grid bias varies inresponse to feed-back, the Miller effect may be utilized to produce Widevariation of the capacitiy indicated at l8 and, therefore, greatlyincreases the feed-back effect and provides a greater degree ofmoduation or control of the oscillator than would otherwise be possibleand a higher signal output or anode current variation.

As a further means for enhancing the modulation or control of the systemdescribed, consideration will now be given to the tuning of the circuit40, which is determined by the inductance at 4| and the shunt capacitiesindicated at 49, 50 and These capacities are determined by the length ofthe cable, the tightness of the twist of the two control leads 45 and46, and the spacing between the leads and the outer shield, which isdetermined by the thickness of the outer insulation used between theleads and the cable.

As hereinbefore referred to, variations of the twisted pair within thecable with regard to spacing from the shield or other movement withrespect to ground or cathode return connection has substantially noeffect upon the frequency of the circuit 40, or upon the balancedfeed-back normally provided by the inductance 4| since both the controlwires are tightly twisted and in movement toward or away from the shieldor other grounded object, provide a simultaneous variation of both thecapacity indicated at 50 and the capacity indicated at 5| in the samesense, so that no unbalance of the feed-back is caused thereby,

Further, since the capacities represented at 5|l-5| are relatively smallcompared to the capacity at 49 between the twisted pair, even if theleads 45 and 46 became slightly separated, slight change in frequencywould result because of movement of the cable within the shield or withrespect to ground or cathode return circuit.

Therefore, it will be seen that the cable may be extended to a distanceof several feet or to a limit in the value of the capacity at 49 whichis required to resonate the coil 4| within a desired frequency range,this range being determined by the desired operating frequency of theoscillator.

Since the transmission line is at all times balanced to ground therebyproviding a balanced positive and negative feed-back of minimum strengththrough the feed-back winding 4|, full modulation control of great rangemay be realized at the terminal end of the transmission line by the useof a relatively small capacity change, such as provided by a movablemetallic reed or spring plate at 51 a small fraction of an inch in widthwhen vibrated almost imperceptibly between two similar narrow fixedplates at 55 and 56 relatively widely spaced therefrom as willhereinafter be described.

To further provide that such small capacity or reactance change maycontrol a large and effective feed-back current, the circuit 25 is tunedsubstantially to resonance with the circuit 40. This is accomplished byadjustment of the variable core 3| in the present example. The exchangeof energy from the circuit 25 to the circuit 40 upon slight movement ofthe controlling element at the modulation source is thereby caused to beinstantaneous and dynamic in effect, because of the resonance conditionexisting between the two circuits. Furthermore, it will be seen as theflow of current through one winding portion increases, the flow throughthe other winding portion decreases proportionately, thereby furtherenhancing the control action or modulation range by push-pull action.

To illustrate the enhanced overall control effect, attention is nowdirected to Figure 4 in which the signal output at the terminal 35 inresponse to tuning of the circuits 25 and I6 is shown by curves l0 andTI, respectively, plotted between frequency and the modulation signalampdtude at the terminal 35. As the core 3| is varied to bring the anodecircuit 25 into resonance with the control circuit 40 at a predeterminedfrequency, such as 1650 kc. for example, the output amplitude increasesalong the curve 70. In operation, the core 3| is adjusted until the twocircuits resonate to provide maximum output as indicated at the point 12on the curve ill, with the grid circuit l6 detuned, for example, to 1600kc., as established on the graph by the line 7 3. The grid circuit I6 isthen tuned to approach the frequency of the anode circuit 25 and at acertain frequency, such as 1620 kc. as indicated at the point 14 on thecurve 1|, a maximum modulation signal ouput is obtained at the terminal35.

The tuning of the grid circuit may now vary about the mean frequency of1620 kc. in response to variations in the grid to plate capacity, 091),as hereinbefore described, while remaining below the frequency of theanode and feed-back controlling circuits 25 and 40 respectively, andthereby permitting full advantage of the Miller effect to be taken inproviding enhanced modulation control of the oscillator. As hereinbeforepointed out, the grid circuit l6 readily may vary in frequency becauseof the loose coupling provided between the windings I! and 4|.

The operation of the system is as follows: With the oscillatorenergized, the anode circuit 25 is tuned to resonance with the controlcircuit 40, and the grid circuit I6 is tuned to resonance at a lowerfrequency of the same order, for maximum response as hereinbeforedescribed. Feed-back of energy from the anode circuit through thegrid-to-anode capacity maintains the grid circuit in a steady state ofoscillation at that frequency, and grid current therefrom flowingthrough the resistor I9 causes the grid to assume substantially a fixednegative bias, which in turn reduces the average anode current flowingthrough the output coupling impedance 33 to a constant normal value, asindicated at 66 in Figure 3. With substantially no variation in averageanode current, the modulation signal output at the terminal 35 is zero.Radio frequency energy from the tuned anode circuit is prevented fromappearing at the output terminal by the bypass capacitor 29 and thechoke coil 34.

Feed-back current at the frequency of the tuned anode circuit 25 flowsfrom the anode connection 21 through the coupling connection 44-43 tothe similarly tuned control circuit 40, and divides at the terminal 42to flow in part upwardly and equally in part downwardly, as viewed inthe drawing, to the terminal ends of the inductance 4| and the controlleads 45 and 46, and thence through the cable to the capacitor plates orelectrodes 55 and 56, respectively, of the modulation or control source.The altemating feed-back currents from each electrode 55 and 56 flow tothe center electrode 5'! and unite to flow back to the ground for thesystem and the anode circuit, through a common path which may be tracedfrom the pivot 59 through the lead 6| to the shield 41, thence throughthe connection leads 48 and 22 to the cathode and chassis ground, andthence through the bypass capacitor 29 to the anode circuit by way ofthe lead 28.. The feed-back currents thus flow through the winding 4| inopposition and when equal, neutralize the feed-back action of the anodecurrent as appears from the following consideration.

Assuming that the flow of feed-back current in the inductance winding4|, upwardly as viewed in the drawing from the terminal 42, excites thecircuit 40 to provide positive feed-back of energy to the grid circuitl6 through the loose inductive coupling, the resulting voltage inducedin the circuit l6 would be in phase with the voltage induced by thenormal feed-back through the grid-to-plate capacity, thereby increasingthe effective circuit voltage and grid current through the grid resistor9.

Likewise, assuming that the flow of feed-back current in the inductancewinding 4|, downwardly as viewed in the drawing from the terminal 42,excites the circuit 40 to provide the opposite or negative feed-back ofenergy to the grid circuit, the resulting voltage induced therein wouldbe in counter phase with the voltage induced by the normal feed-back,thereby decreasing the effective circuit voltage and grid currentthrough the grid resistor.

Assuming a balanced feed-back condition with substantially equalfeed-back current flow through each half of the winding 4| from theterminal 42, by reason of substantially balanced impedance paths 55-51and 56-51 in the control circuit 45-46 connected therewith, as when thecapacitor plate 51 is substantially centered between the fixed plates'55 and 55, it will be seen from the foregoing considerations, that theexcitation effect upon the circuit 40 and feed-back to the circuit IEwill be substantially zero, resulting in no change in grid bias andanode current.

Assuming now a condition of unbalance of the feed-back paths as thecapacitor plate 51 is moved toward the plate 55 and away from the plate55. the impedance or reactance of the balanced paths are variedinversely. The feed-back current flow through the winding 4| from theterminal 42 to the lead 46 will increase and the feed-back current flowthrough the winding 4| from the terminal 42 to the lead 45 will decreasecorrespondingly. Under the conditions assumed, this results in adifferential or preponderance of current flow in the upper half of thewinding 4| and a differential excitation of the circuit 40 to producepositive feed-back on the grid circuit and a corresponding increase innegative grid bias and decrease in anode current, It will be appreciatedthat the effectiveness of this change is en hanced by the inversecontrol or push-pull action, that is, by the simultaneous increase offeed -back current through the one half of the winding 4| and thecorresponding decrease in feed-back current through the other half inresponse to the push-pull variation of impedance at the end of thecontrol circuit.

The change is further amplified by the Miller effect. As the biaspotential becomes more negative, the gain of the tube is reduced and thereflected grid-to-plate capacity, Cgp, which is multiplied by the tubegain and appears effectively at I8, is reduced in value, causing thegrid circuit IE to tune to a higher frequency more nearly approachingthat of the anode and control circuits. This causes the grid circuit tobecome more responsive to feed-back at the anode circuit frequency andadditional circuit voltage and increased negative bias results, therebyproviding a further reduction in anode current, which continues inaccordance with the process outlined above until a steady state ofoscillation once more obtains for the new condition of adjustment of thecontrol circuit.

Furthermore, the exchange of feed-back energy from the circuit 25 to thecircuit 40, and thence inductively to the grid circuit IS in response tochange, however slight, in the balance or static condition of thecontrol circuit at the modulation or control source, is caused to beinstantaneous and dynamic in effect. as referred to hereinbefore. byreason of the condition of resonance existing between the tuned anodecircuit and the tuned control circuit.

Assuming a condition of unbalance of the feedback paths in the oppositedirection, as the capacitor plate 5'! is moved away from the plate 56and toward the plate 55. the feed-back current flow in the upper orpositive feedback portion of the winding 4| is decreased andcorrespondingly increased in the lower or negative feedback portionthereof by reason of the inverse change in impedance in the two branchesof the control circuit. The differential feed-back results in a decreasein the amplitude of the oscillations in the grid circuit and a reductionin the grid bias provided at Ill. The average anode current is increasedcorrespondingly, the gain or amplification factor of the tube isincreased, and the grid circuit frequency is lowered by thecorresponding increase in the reflec ed capacity Cgp at l8, therebyrendering the grid circuit less responsive to feed-back from the anodecircuit and further reducing the negative grid bias until a steady stateof oscillation once more obtains for the new condition of adjustment ofthe control circuit.

The anode current variations thus are intensified and follow thevariations in grid bias provided by the variable dynamic feed-backthrough the tuned circuit 40. which, as has been shown, may becontrolled by inversely varying the impedance of the two branches of thecontrol circuit even minutely or to a relatively fine degree. In thecase of audio frequency modulation provided by the pick-up or controldevice, it will be seen that the grid bias voltage is derived by gridcircuit rectification. The grid potential thus varies at the audiofrequency rate and the amplified audio frequency signal is reproduced bythe corresponding anode current variation in the output impedance 33.The amplified audio frequency 1 1 signal appears at the output terminalas a voltage variation across the output impedance 33.

It will further be seen that the radio frequency energy of theoscillator is amplitude modulated and may be derived from any part ofthe oscillator circuit as at the output lead 36 and utilized in anydesired manner as by conventional amplification and demodulation ashereinbefore noted.

Referring now to Figures 5, 6 and 7 in turn, modifications of themodulation source of Figure 1 are shown in circuit between the controlleads 45 and 46 and ground.

In Figure 5, the leads 45 and 46 are connected respectively to smallspaced co-axial inductance windings I and I6 in series, with a centertap 'I'! connected to the cable 41 and ground, as indicated at I8. Asmall short-circuited conducting ring I9 is arranged to move on a pivot80 between the windings in response to modulating movement of a stylusor other actuating element 8|. As the ring 19 moves, the inductivebalance of the windings is changed inversely to permit a flow ofinstantaneously preponderantly positive or negative feed-back throughthe winding 4| thereby providing amplified modulation of the oscillatorand wide variation of the anode current resulting from the additionalaccelerating effects as hereinbefore described.

In Figure 6. two equal resistance elements 85 and 86 are provided inconnection with the leads 45 and 46 and with a common return path to theshield 41 and ground through a lead 81. As the resistance elements arevaried in a manner to increase the one and decrease the othercorrespondingly in impedance value, the feed-back paths are therebyunbalanced and modulation control results as before described. It hasbeen found that a relatively low resistance of 5000 ohms is satisfactoryfor this purpose in each branch of the control circuit.

In Figure '7. a circuit similar to that of Figure 5 is shown, in whichtwo small coax al inductance coils 90 and 9| are connected in seriesaiding between the leads 45 and 46 and at their mid-point to the cable41 and ground through a lead 92, The inductive unbalance is effected bya, body or particle of matter indicated at 93 wh ch is movable betweenthe coils axially relatively thereto, as a core. by any suitable meansfor effecting modulation. for example, in the same manner as the ring 19in the embodiment shown in Figure 5 or as a particle of foreign mattermoving in either direction, as indicated by the arrowed line 94. in anycarrying medium (not shown).

Thus, the modulation source provided in connection with the extendedtransmission line or control circuit may assume various forms for theutilization of push-pull or inversely variable imped nce or reactancecontrol elements as may be required in the various applications to whichthe invention is adapted.

Referring again particularly to Figure 1, the modulation outputavailable at the terminal 35 may be utilized in any suitable manner andpreferably is taken to any suitable utilization means not shown, througha coupling capacitor 91 connected between the output terminal 35 for theoutput impedance 33 and a modulation frequency amplifier 98 representingany suitable amplifier means for increasing the modulation output energy to a suitable value for utilization.

From the foregoin description, it will be seen that in an oscillatorcircuit embodying the invention, the dynamic control provided by thetuned circuits 25 and 40, in conjunction with the Miller effect upon thefrequency variation of the grid circuit, the push-pull action of thefeed-back control circuit 40 and of the variable reactance or impedanceelement at the end of the transmission line provide a maximum highdegree of sensitivity in response to slight movement of the modulatingor control element of the system.

Referring now to Figure 8, a phonograph record reproduction system isarranged in accordance with the invention, to include a cathode I00, ananode IOI and a control grid I02 for an electronic tube oscillator inthe same envelope I03 with modulation signal or audio frequencyamplifier tube elements comprising a second cathode I04, a second anodeI05, and a second control grid I06, thereby economizing on the number oftubes employed. In the present example, the tube I03 may be of the typeknown on the commercial market as an 68C? tube or dual triode of theindirectly heated cathode type as indicated.

A variable tuning inductance I08 is connected between the control gridI02 and the cathode I00 of the oscillator portion of the tube, and thegrid circuit thus formed is provided with a grid resistor I09 and a gridcapacitor IIO connected between the grid I02 and the inductance I08. Asindicated in the drawing, the inductance I09 is variably tunable by amovable tuning core of ferro magnetic or other suitable materialindicated at III, in conjunction mainly with the reflected shuntcapacity of the tube elements IOI' and I02 or Cop as in the circuit ofFigure 1.

The anode circuit is likewise tunable by a variable inductance II 2 andthe tube capacities effectively in shunt therewith. The inductance isconnected between the anode MI and a signal output resistor II3, whichin turn, is connected with a source of positive anode potentialindicated at H4. The tuning inductance I I2 is variably tunable as inpreceding example, by means of a movable core of ferro magnetic or othersuitable material as indicated at, I I5.

In operation, as in the preceding embodiment of the invention, the gridcircuit is tuned to a slightly lower frequency than the anode circuitand a steady state of oscillation is established by feed-back throughthe grid-to-plate capacity, Cgp. and adjustment of the grid resistorI09.

Modulation feed-back energy is derived from the tuned anode circuit by afeed-back connection I I6 with the anode and is applied to a feedbackcontrolling or modulating inductance, comprising two windings II! andH8, through a coupling capacitor H9. The windings II! and H8 providebalanced sections of a single inductance as indicated and as describedin connection with Figure 2, to provide a balanced feedback inductancewhich is tunable by the distributed capacity of a transmission linecomprising control circuit leads I20 and I2I connected therewith asindicated, the feed-back connection from the anode circuit being appliedto a center tap connection I22 thereon.

The transmission line preferably is contained in a conducting shieldindicated at I 23 and extends to a pick-up device comprising two fixedsubstantial widely separated electrodes I 24 and I25 between which ismounted a flexible laterally movable electrode I26 carrying a stylusI2'I at its free outer end. The inner terminal ends of the electrodesare mounted in a block of suitable insulating material I28 and themovable electrode is connected to the shield and ground through aconnection indicated at I29. Ground 13 connection for the shield isprovided. through a lead indicated at I30, which is connected with aground lead I3I of the oscillator and amplifier circuit, terminating ina negative anode supply terminal indicated at I32.

Modulation of the oscillator in sensitive response to the slightestmovement of the stylus I21 is effected through self-excitation of theoscillator and dynamic control of feed-back from the anode circuit tothe grid circuit through the inductance comprising the sections I I1 andH8 loosely coupled to the grid circuit inductance I08, in conjunctionwith the Miller effect upon the frequency variation or modulation of thegrid circuit, and the push-pull inversely variable reactance action ofthe control circuit.

The modulation or audio frequency output is derived across the gridresistor I09 shunted by the grid or filter capacitor H and appears inamplified form across the output resistor H3 at the output terminalindicated at I35. In the present example, the signal output is appliedto the amplifier control grid I through an audio frequency couplingcapacitor I36 across the impedance of a grid resistor I 31 connected tothe ground lead I3I. The grid resistor is provided with a suitable radiofrequency by-pass capacitor I38.

The amplifier portion of the dual triode is self-biased by means of acathode resistor I39 connected between the cathodes I00 and I04 and theground lead I3I, The cathode bias resistor is provided with a suitableaudio frequency by-pass capacitor indicated at I40.

The output anode I05 of the amplifier portion of the tube I03 isconnected by a lead I4I to the positive supply terminal II 4 through anoutput coupling resistor indicated at I42, across the impedance of whichthe amplified audio freouency output is taken and applied to anyadditional au io frequency or power amplifier, as indicated at I43.through a coupling capacitor I44. The amplifier I43 is, in turn,connected to a sound reproduction device or loudspeaker, indicated atI45, through output leads indicated at I46.

In a practical embodiment of the invention for phonograph recordreproduction, as shown in Figure 8, utilizing a tube of the characterreferred to, having an amplification factor of substantially '70 foreach portion thereof and a grid-to-plate capacity of substantially 2.4mmfd, the grid resistor at I09 was found to provide most satisfactoryoperation when adjusted to substantially 10,000 ohms in value with agrid capacitor of .0002 mfd. capacity at I I0, although a value as highas 1.0 megohm could be used at I09 with satisfactory results. Thefeed-back capacitor I I9 and the by-pass capacitor I38 each may have avalue of .0002 mfd., with an amplifier grid resistor of .5 megohms at I31. The coupling resistors at I I3 and I42 may have any suitab e value,such as 1,000,000 ohms, and the coupling capacitors at I36 and I44 mayeach have avalue of .01 mfd.

The operating frequency in the circuit under consideration was of theorder of 1600 kc. which proved to be satisfactory. The anode circuit wastuned to the frequency of the coupling or control circuit or 1650 kc.,while the grid circuit was tuned to a mean frequency of substantially1620 kc.

In the embodiment of the invention shown in Figure 8, the grid and anodeinductance elements I 08 and I I2 each ma comprise substantially 144inches of litz wire form wound on a diameter insulating tube, while thecontrol inductance I II-IIB may comprise two parallel litz wires eachsubstantially 72" in length, twisted together and form woundsimultaneously on the same size form as the grid and anode inductances,as shown substantially full-size in Figures 9 and 10, to which attentionis directed along with Figure 8.

In Figure 9, the anode coil or inductance is shown at I50 on a form I5I,at one end of which is mounted a metallic cap I52. The cap carries athreaded adjustment screw I53 connected at its inner end with a tuningcore I54 of suitable ferro-magnetic material such a powdered iron mixedwith an insulating binder. Tuning 0r variation of the inductance I 50 isprovided by movement of the core axially with respect thereto as is wellunderstood.

Referring to Figure 10, the grid inductance is provided by a form woundcoil I56 mounted on an insulating tube I51 with the modulation controlinductance also provided by a similar form wound coil I58 substantiallywidely spaced from the grid coil to provide loose coupling for reasonshereinbefore pointed out.

Tuning of the grid coil I56 is effected by means of an adjustableferro-magnetic tuning plug or core I59 adjustable by means of a screwI60 mounted in a threaded cap I 6| on the grid coil end of thesupporting tube I51.

Referring again more particularly to Figure 8, the pick-up devicecomprising the electrodes I24, I25 and I26 is preferably constructedsubstantially as indicated, with a relativel wide outward spread of thefixed electrodes and is represented diagrammatically on a greatlyenlarged scale in order to illustrate the principle of operation.

From an inspection of the illustration, it will be seen that the fixedelectrodes I24 and I25 are curved outwardly at their free ends toprovide a wide separation between them and the flexible center electrodeI28. The extreme excursions of the center electrode are indicated indotted lines at I and I66, Thus, at the extreme limits of its movement,it will be noted that the center electrode is in spaced parallelrelation to the fixed electrode which it approaches, With thisarrangement, the movable electrode moves in a relatively weakelectrostatic field between the fixed electrodes and provides a highdegree of fidelity in the reproduction of sound from a record.Furthermore, operation of the movable electrode from a neutral positionslightly displaced toward either of the fixed electrodes has noappreciable effect upon the fidelity and uniformity of the signal outputfrom the oscillator.

From the foregoing description, it will be seen that a phonograph recordreproduction system in accordance with the invention, may be provided bya minimum number of low-cost circuit elements which may be assembled insubstantfaly a. non-radiating oscillator capable of producing arelatively high gain at the output circuit thereof in response to minutevariations of the control element.

Furthermore, because of the effectiveness of the control system, thepick-up device or control element may comprise an impedance or reactanceof relatively low value and may, therefore, be of small size and weighmerely a fraction of an ounce on a record surface when in use, therebyproviding minimum Wear and minimum pick-up of surface noise inoperation.

While the invention has been shown and described in several of itspresent preferred embodiments, it should be understood that it is notlimited thereto, but may be carried out in other forms and for otherpurposes including those hereinbefore referred to.

What is claimed as new and useful is:

1. In an electronic tube oscillator system, means providing a tunedanode circuit responsive to a predetermined frequency, means providing atuned grid circuit responsive to a frequency of the order of andslightly lower than the first named frequency and having as a tuningelement thereof the reflected grid-to-anode capacity of said tube, athird tuned circuit responsive to the frequency of the anode circuithaving an inductance element coupled with the anode circuit atsubstantially the electrical center of said element and providingfeed-back coupling with the grid circuit, means including saidinductance element providing substantially balanced opposing feedbackpaths from said anode circuit to the ground of said system, means forvarying the impedance of said paths to modulate said oscillator system,and means providing a modulation signal output circuit connected withsaid anode circuit.

2. An electronic tube oscillator system comprising in combination, meansproviding a tunable anode circuit and a tunable grid circuit adapted tobe capacitively coupled and tuned by the interelectrodal capacities ofan associated electronic oscillator tube, means connected with said gridcircuit for providing a grid biasing potential for said tube in responseto oscillations in said circuit, an output anode impedance connectedwith said tunable anode circuit, a third tunable circuit comprising aninductance winding loosely coupled with said grid circuit and having amid-tap connection thereon coupled to the anode circuit to receivefeed-back current therefrom, a pair of control leads connected with theterminals of said inductive winding and extending therefrom in closelyassociated relation to each other providing stray capacity between themfor tuning said inductance to resonance with the anode circuit, meansproviding inversely variable impedance elements between the outerterminal ends of said control leads and ground for said system, meansfor adjusting the tuning of said anode and grid circuits whereby thefrequency of the anode circuit is of the order of and higher than thefrequency of the grid circuit, and means for actuating said variableimpedance elements to provide i v pe ce variation thereof and modulationof said system.

3. An electronic tube oscillator system comprising in combination, meansproviding a tunable anode circuit and a tunable grid circuit adapted tobe capacitively coupled and tuned by the inter-electrodal capacities ofan associated electronic oscillator tube, means connected with said gridcircuit for providing a grid biasing potential in response tooscillations in said circuit, an output anode impedance connected withsaid tunable anode circuit, a third tunable circuit comprising aninductance winding loosely coupled with said grid circuit and having anelectrical mid-tap connection thereon coupled to the anode circuit toreceive feed-back current therefrom, a pair of control leads connectedwith the terminals of said inductive winding and extending therefrom inclosely associated substantially parallelrelation to each otherproviding stray capacity between them for effectively tuning saidinductance to resonance with the anode circuit, means 16 providing acommon ground circuit return path to the grid circuit and anode circuitfor said system, means providing an inversely variable impedance elementbetween said last named circuit means and each of said control leads,means for adjusting tuning of said anode and grid circuits whereby theanode circuit is of the order of and slightly higher than the frequencyof the grid circuit, and means for actuating said variable impedancemeans to provide inverse impedance variation thereof and differentialvariation of feed-back through said inductance winding.

4. In an electronic tube control apparatus, the combination of a tuninginductance adapted'to provide with reflected anode-to-grid capacityeffectively in parallel therewith a tunable grid circuit responsive to apredetermined oscillation frequency, a resistance element in saidcircuit for establishing a variable grid biasing potential in responseto variations in the amplitude of oscillations in said grid circuit, asecond tuning inductance adapted to provide with the inter-electrodaltube capacities effectively in parallel therewith a tuned anode circuitresponsive to an oscillation frequency of the order of and slightlyhigher than that of the grid circuit, a signal output circuit coupledwith the anode circuit, a tuned feedback control circuit having aninductive winding providing feed-back coupling between the anode circuitand the grid circuit, and means for conveying feed-back current from theanode circuit through said winding differentially to providedifferential in-phase and counter-phase inductive feed-back of energy tosaid grid circuit to vary the amplitude of oscillations therein.

5. In an electronic tube control apparatus, the combination of a tuninginductance adapted to provide with reflected anode-to-grid capacityeffectively in parallel therewith a tunable grid circuit responsive to apredetermined oscillation frequency, a resistance element in saidcircuit for establishing a variable grid biasing potential inresponse tovariations in the amplitude of oscillations in said grid circuit, asecond tuning inductance adapted to provide with the interelectrodaltube capacities effectively in parallel therewith a tuned anode circuitresponsive to an oscillation frequency of the order of and slightlyhigher than that of the grid circuit, a signal output circuit coupledwith the anode circuit, a tuned feed-back control circuit having aninductive winding providing feed-back coupling between the anode circuitand the grid circuit, means for conveying feed-back current from theanode circuit through said winding differentially to providediflerential in-phase and counter-phase inductive feed-back of energy tosaid grid circuit to vary the amplitude of oscillations therein, saidlast named means comprising a variable impedance element connected withone terminal of said winding, a second variable impedance elementconnected with the other terminal of said Winding, and means for varyingat least one of said impedance elements with respect to the other,thereby to provide a differential flow of feed-back energy through saidwinding and a corresponding variation in amplitude of grid circuitoscillations.

6. In an electronic tube oscillator system adapted to generateself-oscillations by feed-back, the combination of anode and gridcircuits coupled to provide said oscillations in operation, additionalmeans for feeding back energy from the anode circuit to the grid circuitcomprising an inductance winding coupled to said grid circuitinductively, said winding having two inductively opposed portions, meansfor tuning said por-= tions in series aiding inductive relation to thefrequency of the anode circuit, circuit means connected with the anodecircuit for conducting feed-back current through said winding portionsin opposition to provide simultaneous positive and negative feed-backaction diiferentially in said winding, means for controlling the flow offeedback current through each of said winding portions to vary thedifferential feed-back effect and the strength of oscillations in thegrid circuit, and means for deriving an electrical output effect fromthe anode circuit as a result of said variation.

7. In an electronic tube oscillator system, means providing a tunedanode circuit responsive to a predetermined frequency, means providing atuned grid circuit responsive to a frequency of the order of andslightly lower than the first named frequency and having as a tuningelement the reflected grid-to-anode capacity to said tube, a third tunedcircuit responsive to the frequency of the anode circuit having aninductance element coupled with the anode circuit at substantially theinductive center of said element and providing loose inductive couplingwith the grid circuit, means including said inductance element providingsubstantially balanced opposing feedback paths from said anode circuitto ground of said system, means for inversely varying the impedance ofsaid paths to vary the feed-back effeet on th grid circuit in amplitudeand phase thereby to modulate said oscillator system, and meansproviding a modulation signal output circuit connected with said anodecircuit.

8. The combination with an electronic tube oscillator system having atunable anode circuit and a tunable grid circuit for generatingselfoscillations, of a feed-back winding coupled to said grid circuitand having a coupling connec tion from an intermediate point thereon toone side of the anode circuit, means for tuning said feed-back windinwhereby the anode circuit may be resonated therewith, and a controlcircuit for said winding including a variable impedance elementconnected with each of the terminal ends of said winding and the otherside of said anode circuit, thereby to provide controlled flow offeedback current through said winding in opposite directions from saidintermediate point and differentially positive and negative feed-back onsaid grid circuit in response to variation of said impedance elements.

9. The combination with a self-excited electronic tube oscillator systemhaving a grid circuit and an anode circuit. of means for differentiallyapplying feed-back from the anode circuit to the grid circuit comprisinga feed-back inductance inductively coupled to the grid circuit andcoupled to the anode circuit through a ta connection substantially atits inductive center, a control circuit comprising a pair of leadsconnected with the terminals of said inductance and extending therefromin closely associated relation to each other, a variable impedanceelement connected between each of said leads and ground for saidoscillator system, means for varying at least one of said impedanceelements with respect to the other to vary theflow of feed-back energythrough said inductance differentially thereby to vary the strength ofoscillations in the grid circuit. whereby the anode current iscorrespondingly and proportionately varied, and means connected with theanode circuit for deriving output signal variations in response tovariations in anode current.

10. The combination with an electronic tube oscillator system having atunable anode circuit and a tunable grid circuit for generatingself-oscillations, of a feed back winding coupled to said grid circuitand having a coupling connection from an intermediate point thereon toone side of the anode circuit, means for tuning said feedback windingwhereby the anode circuit may be resonated therewith, a control circuitfor said winding, a fixed capacity electrode connected with each of theterminal ends of said winding through said control circuit, saidelectrodes being spaced, a movable capacity electrode located betweensaid fixed electrodes and providing inversely variable capacity meanstherewith for controlling the flow of feed-back current through saidwinding in opposite directions from said intermediate point anddifferentially positive and negative feed-back on said grid circuit inresponse to variation of said impedance elements.

11. In an electronic tube oscillator system adapted to generateself-oscillations by feed-back, the combination with anode and gridcircuits wherein said oscillations are established in operation, ofadditional means for feeding back energy from the anode circuit to thegrid circuit comprising an inductance winding having two inductivelyopposed portions substantially equally coupled inductively with saidgrid circuit, means for tuning said inductance Winding to the frequencyof the anode circuit, circuit means connected with the anode circuit forconducting feed-back current through said winding portions in oppositionto provide simultaneous positive and negative feed-back actiondiiferentially in said winding, means for variably controlling the flowof feed-back current through each of said winding portions to vary thediiferential feed-back efiect and the strength of oscillations in thegrid circuit, and means for deriving an electrical output effect fromthe anode circuit as a result of said variation.

12. The combination with an electronic tube oscillator system having atunable anode circuit and a tunable grid circuit for generatingselfoscillations, of a feed-back winding coupled to said grid circuitand having a coupling connection from an intermediate point thereon toone side of the anode circuit, means for tuning said feed-back windingwhereby the anode circuit may be resonated therewith, a control circuitfor said winding including a variable impedance element connected witheach of the terminal ends of said winding and the other side of saidanode circuit, thereby to provide controlled flow of feed-back currentthrough said winding in opposite directions from said intermediatepoint, means for varying at least one of said impedance elements toprovide differentially positive and negative feed-back on said gridcircuit in response thereto, and means providing a modulation signaloutput circuit for said system.

13. An oscillator system comprising in combination, an electronic tubehaving an anode, a cathode and a control grid, a tuned anode circuit forsaid tube, a tuned grid circuit for said tube responsive to a frequencyof the order of and slightly lower than the resonance frequency of theanode circuit and having as a tuning element thereof the reflectedgrid-to-anode capacity of said tube, a third tuned circuit responsive tothe resonance frequency of the anode circuit. an inductance element insaid last named circuit provlding substantially loose inductive couplingwith the grid circuit, means providing a coupling connection with theanode circuit at an intermediate point on said inductance element, meansproviding substantially balanced opposing feed-back paths through saidinductance element from said intermediate point to the ground of saidsystem, means for variably unbalancing the impedance of said paths tomodulate said oscillator, and means providing a modulation signal outputcircuit for said system.

14. An oscillator system comprising in combination, an electronic tubehaving an anode, a cathode and a control grid, a tuned anode circuit forsaid tube, a tuned grid circuit for said tube responsive to a frequencyof the order of and slightly lower-than the resonance frequency of theanode circuit and having as a tuning element thereof the reflectedgrid-to-anode capacity of said tube, a third tuned circuit responsive tothe resonance frequency of the anode circuit, an inductance element insaid last named circuit coupled with the anode circuit at substantiallythe electrical center thereof and providing loose inductive couplingwith the grid circuit, means including said inductance element providingsubstantially balanced opposing feed-back paths from said anode circuitto ground of said system, means for variably unbalancing the impedanceof said paths to modulate said oscillator tube, and means providing amodulation signal output circuit for said system.

15. A self-excited electronic-tube oscillator system having a tuned gridcircuit, an anode circuit and means to provide feed-back from the anodeto the grid circuit, comprising a feed-back inductance coupled to boththe grid circuit and to the anode circuit, and having a couplingconnection from an intermediate point. thereon, by

means of which connection coupling to one of 5 said circuits iseffected, and means for conveying feed-back current from the anodecircuit through said inductance diiferentially to provide differentialin-phase and counter-phase inductive feed-back of energy to said gridcircuit to vary the amplitude of oscillations therein, as well ascontrolling the relative resonant frequencies of the circuits byreflected reactance.

16. An oscillator system according to 15, wherein the feed-backconveying means comprise a 5 control circuit including at least onevariable impedance having terminals respectively connected with each ofthe terminal ends of the inductance and with the other side of thecircuit connected to the intermediate point of the inductance.

1'7. An oscillator system according to claim 15, including a pair ofimpedance elements and means for varying at least one of the impedanceelements with respect to the other to provide inverse impedancevariation thereof and modu- 5 lation of the system.

2 PAUL WEATHERS.

REFERENCES CITED The following references are of record in the 30 fileof this patent:

UNITED STATES PATENTS

